I'm not against Behaviors, but I'm still in favor of HitFilm adding something similar to AE expressions. You can essentially create your own behaviors if you know what you're doing. It doesn't have to be just one or the other. Both solutions can exist, with users picking the one they want based on their experience, coding comfort level, etc.

When a plane/car turns what is that? This is an acceleration perpendicular to the motion vector.

So right now we have acceleration in some global/world coordinate space. These suggestions are object relative accel vectors.

A common mograph move is a swing. This is just a rotation from an anchor. We have drag now and we would want the swing/rotate to be damped as well. The existing drag could possibly affect the rotation or the rotation behavior could have a damping factor. I can see that we could want separate movement "friction" and rotation "friction".

A bounce is a more complicated but a simple one like what exists in the particle sim with deflectors could be good enough. Like dropping text/whatever onto a surface. Again, behavior already has a vector so the bounce reflection angle can be determined. Bounce would have a parameter to define the bounce energy. 100% being no loss of energy/velocity. So in a gravity acceleration field and less than 100% bounce energy would should get a natural'ish decay of energy such that the object just sits on the surface/plane.

I agree that it would be great to have full scripting capabilities like in AE, but I think this is a great alternative for now. It's my guess that behaviors are easier to use for the non-coders out there and I'm pretty sure it's been a lot easier to implement than scripting would've been.

Plus the kinds of scripts I'd write if HitFilm supporting true scripting wouldn't necessarily be for expression-like things. Coming from a Maya background, I'd be more inclined to write custom tools/features/utilities.